Friday I was playing around with V.A. and an HP 8903B distortion analyzer.
You might have noticed that there is a THD tab in the analyzer section. Using the 8903B as an external source, I got the number to agree between the HP and V.A., sort of. Both said about 0.002% THD if VA was switched to "None" for window smoothing. Anything else and VA reported much lower THD than the HP did. Of course I was using a soundcard for input to VA and the 8903B's own input.

What was also interesting is that the HP 8903B was measuring THD+n when reporting 0.002% and VA had to be in THD (only) mode to get the same number. I know there are some questions and concerns about how software measures THD, so I thought the comparison was interesting to see the two approaches.

FWIW, here is an ARTA spectrum of the HP distortion as seen by my M-Audio card. Orange is the HP signal, green is the M-Audio DAC itself.
I used ARTA here because it runs faster than VA for FFT plots.

Pano, it would be interesting to also compare the two instruments measuring higher levels of distortion. If you put one or a pair of diodes (in parallel but one reversed) across the source output, you can tweak the level for something like a few percent distortion. Probably good practice to put in some R to avoid embarrassing the source.

I find 5% a good figure - easy to hear and see, and well above the residuals in both systems. If you set the frequency low, you'll get a lot of harmonics in below the sound card upper frequency limit.

I guess the differences between the instruments might be due to such things as:
- the hardware device might have a wider bandwidth and higher self noise?
- the hardware device probably operates by nulling out the fundamental and measuring what's left? (Could you be seeing some residual fundamental?)
- VA operates by measuring the harmonics individually and summing them?
- VA will presumably ignore card artefacts that are not numerically related to the fundamental in the THD measurement, but will include them in the noise section?

I tried it comparing VA with the N&D set I built back in the 70's. 5% on the N&D set read as 4.86% on VA. That's probably within the accuracy of the N&D set's analogue meter!

Yes, easy.
There is a button on the bottom of the window where you can save the .wav, and burn it on CD or so.
All you have to do is to start replay at the correct moment, but RMAA is not picky, it somehow recognises when the test signals are starting.